Understanding Size, Power, Components, and Cost
By David Cook
The first plasma arc cutting (PAC) systems, developed in the Sixties, were thousand amp
monsters designed to blast through 6 inch thick stainless steel. Their mechanized torches
were moved by XY cutting machines and powered by DC units the size of refrigerators.
Surprisingly, the PAC industry evolved from high to low amp systems, water to gas cooled,
and from gas to air cooled. Today’s handheld air PAC systems are lightweight, portable, and
relatively powerful for their size. They are used for cutting everything from thin gauge metals
to 1 inch plate. More traditional console PAC systems also are available to handle cutting
tasks up to 2 inches and more. Handheld PAC systems are now the fastest growing segment
of the PAC market because they offer a fast, efficient, and affordable way to cut. This article
offers an overview of manual PAC technology from the early days to the present, including
an explanation of different power supplies, recommendations for selecting and sizing a
system, and other functions and features to look for in a handheld system. Regardless of the
size, all PAC systems contain the same basic components, including a gas supply, DC
power supply, and plasma torch. The torch requires a circuit to initiate an arc and a cooling
Gas Supply
Most older plasma systems used nitrogen as the plasma gas and air or CO2 as the
secondary gas, which required expensive bottles or bulk containers. Now, most handheld
systems use clean, dry shop air to cool the torch and provide the necessary plasma gas.
Shop air currently is the most affordable and versatile plasma gas. It is readily available and
provides good cut edge quality on mild and stainless steel and aluminum. With the exception
of special applications, such as thick stainless steel and aluminum cutting or plasma
gouging, almost all handheld systems today use air plasma. Several manufacturers even
have developed air plasma systems with small, onboard air compressors.
Power Supplies
PAC power supplies are direct current electrode negative (DCEN). The process requires a
constant source of DC and a high open circuit voltage (OCV) to initiate the arc. The following
is a summary of some basic differences among PAC power supply types.
DC Droopers. Early plasma systems included “drooper” power supplies, named for their
drooping output power curves. These units provided a high OCV and relatively stable current
and operating voltage. They used a fixed output DC rectifier bridge consisting of a series of
diodes to convert AC power from a transformer into usable DC for the cutting process. These
simple systems created a lot of power but wasted energy and had too much “ripple” (a
fluctuation in DC output that causes a rough cut and short part life) in their output power. To
further regulate power output, multiple transformers could be used, each providing a higher
level of output current.
Reactors. Reactor power supplies were the next step in power regulation. These used a
reactor device to control the amount of AC voltage supplied to the bridge rectifier. The
reactor consisted of a group of AC coils with a DC winding around it. The current in the DC
winding controlled the amount of AC that passed through the reactor, which created an
adjustable transformer that allowed variable DC output from the bridge.
SCRs. Silicon controlled rectifiers (SCRs) are another type of continuously variable output
power supply. SCRs convert three phase AC power from a transformer directly to DC. They
require huge capacitor banks and large transformers. SCRs are large and powerful and are
used for high amp PAC systems but are not well suited for handheld systems.
Switchmode. Switchmode power supplies use transistors to modulate DC power after the
Choppers. Choppers are a type of switch mode power supply that use power semiconductor
devices such as isolated gate bipolar transistors (IGBTs)—which take raw DC with ripple and
chop it up, rapidly switching the power on and off to smooth the output characteristics. IGBTs
can be fired much faster than old reactor type power supplies. The result is a very smooth
output power curve.
Inverters. Inverters are another type of switchmode power supply. They use devices such as
transistors on the input side of the power train to raise the frequency of the AC into the
transformer. Higher frequency input allows a much smaller transformer to be used. Because
a smaller transformer is used, inverters are much lighter and more portable than
conventional power supplies, making them ideal for handheld applications. Early inverter
power supplies were limited by low output current and complicated design and poor
reliability. When problems occurred,sophisticated techniques and troubleshooting were
required to solve them. Today’s inverters are
more reliable, robust, and powerful. Most manual
PAC systems now use inverter or switchmode
technology. These sophisticated, electronically or
When looking for a new
microprocessor controlled devices are better able
handheld system, consider
to tolerate variations in line voltage, take more
answers to the following
abuse in the field, and deliver better cutting
performance while consuming less power.
• What type of power supply
does the system have? What
All plasma torches contain the same basic
is its capacity?
elements, including:
• Will the system generate
• Electrode to carry the negative charge from
enough power to meet my
the power supply
• Gas distributor, or swirl ring, to spin the
• How much will it cost to
plasma gas into a stable, swirling vortex
• Nozzle to constrict and focus the plasma jet
• Is the torch durable and
ergonomically designed?
The torch is primarily a holder for the consumable
Does it have the latest safety
parts. Torch improvements have been aimed at
optimizing the torch and consumable designs to
• How long is the warranty?
improve cooling, enhance starting characteristics,
• How easy is it to add
and increase cutting capacity. Improvements also
technical support once the
have been made in material selection for
warranty is expired?
consumables and torches to improve durability,
• Can I try before I buy?
such as using high temperature durable plastics
in place of ceramics. Ergonomics have improved
with features such as trigger torches, better handle designs, and options for torch angle or
adjustable torchheads. Safety improvements include parts in place (PIP) circuits and
switches or triggers to prevent the torch from firing without the parts properly installed and
the operator ready. Most handheld systems on the market today use one of two methods to
initiate the plasma arc. The tried and true method is a high frequency (HF) starting circuit
built into the power supply. This system uses a high voltage transformer (similar to a bug
zapper), capacitors, and spark gap assembly to generate a high voltage spark at the torch.
The spark ionizes the plasma gas, enabling current to flow across the air gap between the
nozzle and electrode. The resulting arc is called the pilot arc. High frequency starting
systems are simple, relatively dependable, and require no moving parts in the torch.
However, they do need periodic maintenance to prevent hardstarting problems. Another
potential problem is that high frequency radiates from the system, creating electrical noise
that may interfere with sensitive electronic equipment. Contact start torches use a moving
electrode or nozzle to create the initial spark that enables the pilot arc. When the torch is
fired, the electrode and nozzle are in contact in a short circuit, something called a “dead
short”. But as the gas enters the plasma chamber, it blows the electrode back or the nozzle
forward, creating a spark. This process is similar to the spark created when you quickly yank
an electrical plug out of a live outlet. Contact start torches produce much less electrical noise
than HF systems. These also are “instant on” torches, which reduce cycle time because of
the lack of pre-flow.
Sizing a System
The machine should have sufficient power to handle typical cutting tasks with ease, and it
should be able to cut the material at about 20 inches per minute (IPM) or faster. When an
operator becomes accustomed to the speed of PAC, it is possible to hold line profiling at 70
to 80 IPM. Even faster speeds are possible with template cutting or cutting accessories such
as circle cutters and rolling plate followers. Before purchasing a system, the three material
considerations are:
• Types of materials to be cut
• Thickness of materials to be cut
• Most commonly cut material thickness
The third consideration is the most important when selecting a plasma system. Often, errors
are made in sizing a system for an application, and too little or too much power is acquired
for the most common cutting task.
Underpowering, or trying to cut at the high end or over the system’s cutting capacity, will lead
to poor cut quality, low cut speeds, and high torch and part consumption. Over powering can
lead to cut quality problems, such as heat distortion, wide kerf, and low speed dross.
Generally, more power is better, especially because most systems now allow variable output
so that the power can be dialed down for thinner materials. Figure 1 includes some basic
cutting capabilities for a range of different amperages.
Equipment manufacturers rate the cutting power of a PAC system with a thickness or
capacity rating. These ratings are based on carbon steel and list the thickest metal that the
system will cut with reasonable speed and cut quality, from an edge start. In an edge start,
the operator fires the torch with the nozzle just over the edge of the plate, then begins
cutting. For a pierce start, the operator fires the torch over the plate and blows a hole through
the material before cutting. Piercing through material requires more power and operator skill
than edge starting. For these reasons, the pierce rating, or piercing capacity, usually is half
the cutting capacity. For example, most 100 amp systems will cut 1 inch plate from an edge
start, but can only pierce ½ inch plate.
Some manufacturers also offer a recommended capacity, which is a more useful
specification than maximum capacity. The recommended capacity is the optimum thickness
for the machine in terms of quality, parts life, cut speed, duty cycle, overall productivity, and
cost of operation. If the system doesn’t have a recommended capacity rating, an analysis
can be made by referencing the cut charts or a cut speed curve as shown in Figure 2. The
regularly cut material should fall somewhere in the middle of the cut chart, and the
corresponding speed should be at least 20 IPM.
Cost of Operation
Many variables contribute to the overall cost of operation for PAC, including labor, power,
duty cycle, gas, shop air maintenance, consumables, consumables life, speed of cut, and the
amount of cleanup or secondary operation required. The two most important factors to
consider when purchasing new equipment are consumable cost and consumable life.
Because the part life of different systems varies, consumable cost alone is not the best
measure of a system’s cost of operation.
Consumable Cost. The total consumable cost divided by the consumable life in hours of arc
on time per hour, is the most useful measurement. For example, if the cost of a nozzle is $4,
the cost of the electrode is $6, and together the set lasts 2.5 arc hours, then the cost per
hour, or CPH, is $10 ($4 + $6) divided by 2.5 for a total CPH of $4. Just the nozzle and
electrode are used for this calculation because the other consumable parts are designed to
last much longer. To calculate CPH for all torch components, a weighted average should be
used based on usage ratios. Typically, shields, swirl rings, and caps outlast nozzles and
electrodes in a minimum 20 to1 ratio.
David Cook is director of engineering for Hypertherm’s Centricut brand.
Reprinted with permission from Practical Welding Today®